1. Field of the Invention
The present invention relates to inertial multisensor navigation units (IMU's) for short range, relatively low-accuracy guidance applications, such as munitions. More particularly, this invention pertains to a multisensor in which the rotors are driven by means of an outlying ring of piezoelectric composition.
2. Description of the Prior Art
IMU's measure space-dependent accelerations and rotation, or angular, rates with respect to orthogonal space axes. Their design is beset by numerous complexities, as it requires the simultaneous measurement of six independent variables. For example, gyroscopes of the ring laser type require a lasing cavity dedicated to each input axis. This mandates a total of three lasing cavities, an expensive undertaking, to obtain three of the six required measurements. (An example of a laser device for measuring rotation about three axes is shown in U.S. Pat. No. 4,795,258 of Graham Martin, property of the assignee herein, entitled "Nonplanar Three-Axis Ring Laser Gyro With Shared Mirror Faces".) IMU's employing spinning wheel gyros must deal with their limitation to measurement of rotation with respect to two axes, necessitating the use of an additional drive and servo-mechanism for the third and (redundant) fourth input axes. Again, this does not account for the additional complexity introduced by the remaining acceleration measurements.
Simplicity and economy are particularly significant in the design of IMU's for munitions guidance and like applications. Such uses are characterized by non-reusable payloads, limited flight durations and only moderate accuracy requirements. One economical type of system for measuring both rotation rates and linear acceleration with reference to a set of three orthogonal axes is the multisensor mechanism disclosed, for example, in the following United States patents, alsco the property of the assignee herein: U.S. Pat. No. 4,996,877, entitled, "Three Axis Inertial Measurement Unit With Counterbalanced Mechanical Oscillator"; U.S. Pat. No. 5,007,289, entitled, "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator"; and U.S. Pat. No. 5,065,627 entitled, "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator". The devices disclosed in the above-referenced patents employ piezoelectric drive mechanisms to drive a pair of counterbalanced platforms to oscillate out-of-phase about a common axis within a housing or case. Accelerometers, housed in a vacuum to avoid the effects of gas damping, are mounted at tilted attitudes (for measuring variables in orthogonal planes) with respect to radially-directed elements of the platforms to provide measures of both linear acceleration and rotation. The latter (rotation) values are derived from the (Coriolis) forces sensed by the accelerometers at the resonant frequency of the counter-oscillating structure.
In order to effect the necessary oscillatory motion, piezoelectric elements are fixed to opposed surfaces of webs of drive elements, the ends of which are fixed to the case of the multisensor, that radiate from a central hub. The distal ends of pairs of webs are joined by arcuate elements. The inner portions of rotor arms that support the accelerometers are fixed to the hub so that induced rotation of the central hub results in the rotation or oscillation of the rotor-fixed accelerometers required for measurement of Coriolis forces. The piezoelectric elements are appropriately-poled so that an input drive signal simultaneously induces compression and tension at the opposed web surfaces to cause their bending or flexing (in alternating directions) to produce oscillation of the rotor hubs.
The above-described method for driving the rotors at the self resonant frequency and at a set amplitude (established through the use of pickoff electrodes that feed back oscillation amplitude) is subject to a number of disadvantages. An approximately twenty (20) volt signal amplitude is required to drive the multisensor rotors to achieve an acceptable twenty (20) inches per second peak tangential velocity (a more desirable peak tangential velocity would be in the range of forty (40) inches per second). Additionally, multisensors configured as described above require a multiplicity of piezoelectric elements (and associated electrodes). A current design requires twenty-four (24) piezo elements--two for each of twelve drive webs or beams. The installation and wiring of such a multiplicity of piezo elements is both complex and costly. Finally, it has been found that such designs have difficulty in achieving the type of start-up times (on the order of one second) required to operate at self-resonance with high Q (approximately 1,000).